The present invention relates to a circular arc illumination apparatus such as reflection type of projection aligner, and more particularly to a circular arc illumination apparatus for exposing a large substrate such as a liquid crystal display device having a large screen.
FIG. 4 shows the projection optical system of a conventional 1:1 reflection type of projection aligner comprising a concave mirror 1 and a convex mirror 2. A mask 3 is irradiated by circular arc slit illumination light 4 so as to project the pattern of the mask 3 on a substrate 5. The mask 3 and the substrate 5 are scanned in a direction shown by an arrow B and the direction opposite thereto. The circular arc slit illumination light is generated by enlarging the image produced by light from a circular arc lamp or by using a symmetrical reflecting mirror to scan light produced by a point light source. The latter optics are effective when using ultraviolet rays. This kind of optical system is disclosed in Japanese Laid-Open Patent Publication Nos. 54-123877 and 57-20012. The apparatus disclosed in the former publication has a spherical mirror, and the apparatus disclosed in the latter publication has a four-way curved surface mirror constituted by rotating an elliptical mirror. FIG. 5 shows a circular arc illumination apparatus for obtaining circular arc illumination. The apparatus comprises reflecting mirrors 10, 11, and 12 serving as a spherical mirror and a four-way curved surface mirror; a point light source 7; a slit 8; a spherical reflecting mirror 9; and a pin hole 13.
The conventional apparatuses have the following three disadvantages. The first disadvantage is a lack of uniformity in the illumination. FIG. 6 shows discharge electrodes 14 and 15 of the point light source 7. The emission of the point light source 7 is generated by an arc 7a produced between the electrodes 14 and 15. The discharge position on the surface of the electrode 15 changes with the lapse of time. As a result, the position of the spot light source is nonuniform. Accordingly, the image of the point light source 7 formed via the reflecting mirror 10 shown in FIG. 5 varies, which results in nonuniform illumination. The second disadvantage is inefficiency in the utilization of light because a luminous flux is shaped by the circular arc slit. The third disadvantage is cost and difficulty in manufacture. With respect to the process of exposing a large liquid crystal substrate, which type of substrate has been the subject of research and development in recent years, there are demands for large optical instruments such as a large reflecting mirror which produces a large quantity of circular arc illumination light. Therefore, the cost for manufacturing the apparatus is inevitably high and further, it is difficult to manufacture the apparatus.